Cation exchange membrane and use

ABSTRACT

A fluorinated cation exchange membrane made using reinforcement of oriented, hydrolyzed fabric of a cation exchange copolymer, the fabric having a coating of a melt-processible precursor or derivative of a fluorinated cation exchange resin on at least one surface or throughout.

Fluorinated cation exchange membranes are widely used in theelectrolysis of alkali metal chlorides, among other uses. Lowervoltage--i.e., lower membrane resistance--and higher current efficiencyare greatly desired, because these two factors determine the amount ofelectrical power required for the electrolysis.

Wet cation exchange membranes have rather poor tear strength, and nearlyall of them are now fabric-reinforced, though considerable work has beendone with unreinforced films and with fibril-reinforced films. Thereinforcement is usually made of perfluorinated polymers, such ashomopolymers and melt-fabricable copolymers of tetrafluoroethylene(TFE), because they have enough chemical resistance to withstandexposure to caustic and chlorine. In some cases, as taught in U.S. Pat.No. 4,437,951, sacrificial yarns may also be used for specialadvantages.

Perfluorinated fabrics have some disadvantages, since they block thestraight flow of alkali metal cations through the membrane, increase theresistance of the membrane, and result in uneven current distributionwithin the membrane. For this reason, open fabrics have been generallypreferred to minimize membrane resistance, which raises cell voltage andpower consumption. Also, when a melt-processible precursor to a cationexchange resin, which usually contains pendant groups ending in --SO₂ For --COOR groups, is laminated to a non-functional fabric, thefunctional polymer expands during hydrolysis and use, while thereinforcement does not. This can result in puckering of the film in the"window" areas between reinforcing yarns. This puckering is undesirablebecause it makes leak-tight sealing of the membrane in an electrolysiscell more difficult and provides recesses and protrusions that can trapgas which hinders ion flow.

Some consideration has been given to replacing the perfluorinated fabricwith a cation exchange resin, but no really feasible process has yetbeen disclosed for making such a membrane.

U.S. Pat. No. 3,985,501 describes the melt spinning of an orientedfilament of a perfluoropolymer with side chains terminating in --SO₂ Fgroups, followed by weaving into a liquid-impermeable fabric, followedby hydrolysis to the cation exchange form. This fabric was not used forreinforcing a cation exchange film laminate. When a low denier orientedunhydrolyzed yarn such as this is used to weave a fabric, the elasticityof the yarns causes the yarns in the fabric to become shorter when thefabric is removed from the loom, causing the fabric to pucker into acrepe fabric and causing further shrinkage in the laminator. Such afabric with built-in strain is undesirable for use in reinforcing acation exchange membrane.

Japanese Laid-open Application No. J57/25330 replaces part but not allof the non-functional perfluorocarbon polymer yarns with unorientedcation exchange yarns. The cation exchange yarns do not strengthen thefabric but only stabilize it against distortion. The only example inthis application gave a current efficiency of 94% and anelectrode-to-electrode voltage of 3.46 volts.

In copending application 07/316,630, now U.S. Pat. No. 4,964,960, lowervoltage is achieved by using a reinforcing fabric made of oriented andhydrolyzed yarn of material similar to that of the membrane to bereinforced. With such yarns and fabric, improved adhesion or bondingbetween the hydrolyzed yarn and the melt-processible film as well asimproved fabric stability, that is, resistance to shifting of fibersduring handling prior to lamination, are desired.

Bonding is important when the membrane is hydrolyzed, causing themelt-processible resin to swell. Without sufficient adhesion of thefabric to the film, the film will pull away from the fabric, leavingfree space. Reduction or elimination of any free space between thereinforcement and the film after final hydrolysis means less or novolume of liquid in the membrane. Such liquid regions distort currentflow during operation, because they are more conductive than thepolymer. On shutdown, chlorine and hypochlorite in the liquid regionscannot be easily flushed out. These chemicals can diffuse toward thecathode, attacking it and allowing ions from the cathode metal to beabsorbed by the membrane, undesirably raising cell voltage.

Stability is important since shifting of fibers during handling resultsin a final membrane of less than fully satisfactory properties. Withshifting, the membrane would have areas that are unreinforced or poorlylaminated. Such a membrane would suffer from uneven current distributionin use. The fabric in such a membrane may be thick in places leading to,during lamination, incomplete encapsulation and resulting leakage ofelectrolyte.

In the present invention, the oriented, hydrolyzed yarn is coated withmelt-processible copolymer or superficially converted tomelt-processible form prior to being made into a fabric for laminationinto a membrane. Alternatively, a fabric is made from oriented,hydrolyzed, fluorinated cation exchange resin and then coated withmelt-processible copolymer or superficially converted tomelt-processible form. The melt-processible coat or sheath readily bondsto the melt-processible film with which the fabric is laminated to makea membrane, thus improving adhesion.

In the present invention, the yarns are better for weaving into a stablefabric than un-coated yarns because coated yarns readily bond to eachother, preventing shifting of the yarns in the fabric during handling.

With the present invention, tightly-woven fabrics may be used forstrength, because after final hydrolysis the entire structure will beion-conductive. Also, since the fabric has improved wettability by filmsof melt-processible copolymers used in making membranes, tighter-wovenfabrics may be used without encountering problems due to voids in thelaminates.

SUMMARY OF THE INVENTION

A coated cation exchange yarn (also referred to herein as sheath/coreyarn) has been invented. It has, after hydrolysis, a denier between 50and 400 grams per 9000 meters (g/9000 m). It comprises a core that is anoriented fluorinated cation exchange resin, oriented, preferably bybeing stretched or drawn at least 1x, sufficiently to provide a minimumtenacity after hydrolysis of 0.5 grams per denier (g/denier). On thecore is a coating (sheath) that is a melt-fabricable precursor to afluorinated cation exchange resin. Both the sheath and core have a moleratio of non-functional:functional monomer of 2.8-11.8:1, the ratio forthe sheath and the ratio for the core being within three units,preferably one unit, of each other. A cation exchange fabric may bemade, preferably by weaving or knitting, from this sheath/core yarn.

A coated cation exchange fabric has been invented. It comprises a basefabric that is an oriented, hydrolyzed, fluorinated cation exchangeresin in which the mole ratio of non-functional:functional monomer is2.8-11.8:1. On the base fabric is a thin layer of a melt-fabricableprecursor to the fluorinated cation exchange resin (coating) covering asufficient portion of the base surface to assure good adhesion of thefabric to a film when laminating the film to the fabric to make amembrane. The coating may also be throughout the base such as when thebase is made from coated yarn. The coating has anon-functional:functional monomer ratio that is within three units,preferably 1, of the ratio of the base fabric.

The yarn or fabric can be made by coating the oriented, hydrolyzed yarncore or fabric base with a solution, dispersion, or other liquidcomposition of the precursor resin, followed by removal of volatiles.For uniform or easier coating, it may be desirable to use low viscosityor high solids solutions of melt-fabricable polymer, prepolymer oroligomer which can be cross-linked, post-polymerized or otherwise curedwith peroxides, heat, light or radiation.

The yarn or fabric can be made by making a yarn or fabric of orientedresin with --COOH groups on the pendant side chains by hydrolyzing ahydrolyzable precursor in oriented yarn form, converting it into afabric if desired, and then esterifying the surface of this yarn orfabric to make a melt-processible sheath.

The yarn or fabric can be made by making a yarn core or fabric base oforiented resin with alkali metal sulfonate groups on the pendant sidechains by hydrolyzing a hydrolyzable precursor in oriented yarn form,converting it into a fabric if desired, and then converting the surfaceof this yarn or fabric to make a melt-processible sheath or coating with--SO₃ H, --SO₂ Cl, or --SO₂ F end groups or end groups of a fusiblesulfonate salt.

The fabric can be melt-laminated with at least one film of at least onemelt-processible fluorinated cation exchange resin precursor in whichthe non-functional:functional group ratio is within three units,preferably one unit, of that of the sheath or coating resin.

After hydrolysis, the membrane may be used as an ion-exchange membraneparticularly to electrolyze alkali metal halide solution to make halogenand alkali metal hydroxide.

Because of the oriented core, the laminate has enough stiffness andstrength, particularly tear strength, to be handled during preswellingand mounting in a cell. Because the sheath has good adhesion to the coreand to the film or films, the final laminate has good integrity and doesnot delaminate even during the swelling that accompanies hydrolysis.

DETAILS OF THE INVENTION

Core and sheath sections of a cation exchange yarn, which are useful,among other things, in making reinforcing fabrics to be laminated withfilms to make membranes, may be made of carboxylic polymers, sulfonylpolymers or a combination of the two. The same polymers may be used tomake base fabrics and their coatings as well as the films which arelaminated to the fabric to make membranes. It is preferred that thelayer of the membrane which is mounted next to the catholyte be made ofa carboxylic polymer.

The carboxylic polymers with which the present invention is concernedhave a fluorinated hydrocarbon backbone chain to which are attached thefunctional groups or pendant side chains which in turn carry thefunctional groups. When the polymer is in melt-fabricable form, thependant side chains can contain, for example, ##STR1## groups wherein Zis F or CF₃, t is 1 to 12, and W is --COOR or --CN, wherein R is loweralkyl. Preferably, the functional group in the side chains of thepolymer will be present in terminal ##STR2## groups wherein t is 1 to 3.

The term "fluorinated polymer", as used herein, for carboxylic and forsulfonic polymers, means a polymer in which, after loss of any R groupby hydrolysis to ion exchange form, the number of F atoms is at least90% of the total number of F, H, and Cl atoms in the polymer. Forchloralkali cells, perfluorinated polymers are preferred, though the Rin any COOR group need not be fluorinated because it is lost duringhydrolysis.

Polymers containing ##STR3## side chains, in which m is 0, 1, 2, 3 or 4,are disclosed in U.S. Pat. No. 3,852,326.

Polymers containing --(CF₂)_(p) COOR side chains, where p is 1 to 18,are disclosed in U.S. Pat. No. 3,506,635.

Polymers containing ##STR4## side chains, where Z and R have the meaningdefined above and m is 0, 1, or 2 (preferably 1) are disclosed in U.S.Pat. No. 4,267,364.

Polymers containing terminal --O(CF₂)_(v) W groups, where W is asdefined above and v is from 2 to 12, are preferred. They are disclosedin U.S. Pat. No. 3,641,104, U.S. Pat. No. 4,178,218, U.S. Pat. No.4,116,888, British No. 2,053,902, EP No. 41737 and British No.1,518,387. These groups may be part of ##STR5## side chains, where Y=For CF₃ or CF₂ Cl. Especially preferred are polymers containing such sidechains where v is 2, which are described in U.S. Pat. No. 4,138,426 andU.S. Pat. No. 4,487,668, and where v is 3, which are described in U.S.Pat. No. 4,065,366. Among these polymers, those with m=1 and Y=CF₃ aremost preferred.

The above references describe how to make these polymers.

The sulfonyl polymers with which the present invention is concerned arefluorinated polymers with side chains containing the group ##STR6##wherein R_(f) is F, Cl, CF₂ Cl or a C₁ to C₁₀ perfluoroalkyl radical,and X is F or Cl, preferably F. Ordinarily, the side chains will contain--OCF₂ CF₂ CF₂ SO₂ X or --OCF₂ CF₂ SO₂ F groups, preferably the latter.For use in chloralkali membranes, perfluorinated polymers are preferred.

Polymers containing the side chain ##STR7## where k is 0 or 1 and j is3, 4, or 5, may be used. These are described in British No. 2,053,902.

Polymers containing the side chain --CF₂ CF₂ SO₂ X are described in U.S.Pat. No. 3,718,627.

Preferred polymers contain the side chain ##STR8## where R_(f), Y, and Xare as defined above and r is 1, 2, or 3, and are described in U.S. Pat.No. 3,282,875. Especially preferred are copolymers containing the sidechain ##STR9##

Polymerization can be carried out by the methods described in the abovereferences. Especially useful is solution polymerization using ClF₂CCFCl₂ solvent and (CF₃ CF₂ COO)₂ initiator. Polymerization can also becarried out by aqueous granular polymerization as in U.S. Pat. No.2,393,967, or aqueous dispersion polymerization as in U.S. Pat. No.2,559,752 followed by coagulation as in U.S. Pat. No. 2,593,583.

The copolymers used herein should be of high enough molecular weight toproduce films which are self-supporting in both the melt-fabricableprecursor form and in the hydrolyzed ion-exchange form.

A membrane having at least one layer of a copolymer having sulfonylgroups in melt-fabricable form and a layer of a copolymer havingcarboxyl groups in melt-fabricable form, such as made by coextrusion,can be used as one of the component films in making the membrane of theinvention. Such a laminated structure may be referred to as abimembrane. Preparation of bimembranes is described in JapaneseLaid-open Application No. K52/36589, published as J83/33249.

The customary way to specify the structural composition of films ormembranes in this field is to specify the polymer composition, theion-exchange capacity or equivalent weight or ratio of nonfunctional tofunctional monomer, and thickness of the polymer films inmelt-fabricable form from which the membrane is fabricated. This is donebecause the measured thickness varies depending on whether the membraneis dry or swollen with water or an electrolyte, and even on the ionicspecies and ionic strength of the electrolyte, even though the amount ofpolymer remains constant.

For use in ion-exchange applications and in cells, for example achloralkali cell for electrolysis of brine, the membrane should have allof the functional groups converted to ionizable functional groups. Thesewill be sulfonic acid and carboxylic acid groups, or preferably alkalimetal salts thereof. When the term "sulfonic ion-exchange groups" isused, it includes not only the sulfonic acid group but particularly thealkali metal salts thereof. Similarly, the term "carboxylic ion-exchangegroups" means the carboxylic acid group and particularly the alkalimetal salts thereof. The alkali metals preferred for use in thisinvention are potassium and sodium, particularly sodium, which leads tothe production of sodium hydroxide.

Conversion to ionizable functional groups is ordinarily and convenientlyaccomplished by hydrolysis with acid or base, such that the variousfunctional groups described above in relation to the melt-fabricablepolymers are converted respectively to the free acids or the alkalimetal salts thereof. Such hydrolysis can be carried out in an aqueousbath of mineral acid or alkali metal hydroxide. Hydrolysis in thehydroxide is preferred as it is faster and more complete. Use of hothydrolysis baths, such as near the boiling point of the solution, ispreferred for rapid hydrolysis. The time required for hydrolysisincreases with the thickness of the structure. It is advantageous toinclude a water-miscible organic compound such as dimethyl sulfoxide inthe hydrolysis bath, to swell the membrane to increase the rate ofhydrolysis.

The carboxylic and sulfonic polymers used in the invention for theyarns, the fabrics and the films which are used in making membranes havea mole ratio of non-functional:functional monomer of 2.8-11.8:1,preferably 5.3-7.5:1. TFE typically is the non-functional monomer usedand will therefore be generally used throughout this specification.Other non-functional monomers are considered to be substitutable for TFEin all cases referring to TFE alone. If the ratio is lower than 2.8:1,the copolymer will tend to be soft and difficult to handle and have anexcessively high water absorption. If the ratio is higher than 11.8:1,the membrane will have a high resistance (high voltage).

The yarn, fabric and film for a particular laminate should have endgroups selected from sulfonyl end groups and carboxyl end groups.Sulfonyl includes the alkali metal salt, fusible salts, --SO₂ F, --SO₂Cl, and --SO₃ H end groups. Carboxyl includes --COOH, --COOR where R islower alkyl (that is, C₁₋₅ alkyl), --CN and carboxyl salts.

The mole ratio of TFE:functional monomer should not vary by more than 3units, preferably 1 unit, between the core and sheath or between thesheath and the film laminated to the fabric.

Those skilled in the art will realize that the optimum mole ratio willdepend on the structure of the polymer or polymers used in the membrane.For example, if the pendant side chain containing the functional groupis short, relatively high mole ratios may be used. If the thickness ofthe membrane is at the high end of the range mentioned, the resistancewill be relatively high, and low mole ratios will be desired. If themembrane is carboxylic rather than sulfonyl, the resistance will berelatively high, and low mole ratios will be desired. If caustic outsidethe 30-35 wt. % range is desired, high mole ratios in the range7.5-8.5:1 may be preferred.

The core yarn may be monofilament or multifilament. The sheath is placedpartially or entirely around the core.

The yarns may be of ordinary round cross-section or may have specializedcross-sections. Oblong or rectangular cross-sections, if suitablyoriented in a fabric to the membrane, make it possible to get morereinforcing action with a thinner overall membrane.

The core yarns must be oriented before hydrolysis, but the orientationprocess is not critical. Orientation can be done by drawing duringspinning as the yarn comes from the spinneret. Yarn may also be orientedby drawing in a separate step after spinning is finished. While beingstretched in a separate step, the yarn is heated on a hot surface,through a heated tube, or by passing through steam. The amount ofstretching or drawing should not be so great as to cause breaks inconstituent filaments of multifilament yarns, and is normally 2 to 4times the unstretched length. The degrees of orientation and hydrolysisshould be sufficient to give the core yarn a minimum tenacity of 0.5g/denier and a minimum tensile modulus of 2 g/denier after hydrolysisand superficial drying. Higher tenacities and tensile moduli arepreferred. If the tenacity is too low, breaks may occur during thecoating process or on the loom. If the tensile modulus is too low, thecore yarn will be difficult to feed into the coating die, especiallywhen starting a run, and too elastic to weave well on the loom.

Hydrolyzed cation exchange yarn is several times as stiff asunhydrolyzed yarn made of the precursor resin, as shown in the Examples.

The coated cation exchange fabric may be made, among other ways, fromhydrolyzed yarns by any known method such as weaving or knitting. Whenwoven, fabrics may be of various weaves, such as the plain weave, basketweave, leno weave, or others. The yarns may be sheath/core yarns thusimparting the coating throughout the fabric or the fabric base formedmay have the coating added after being formed into a fabric.

When woven, the weave used is not critical and the maximum yarn count isnot critical because the yarn becomes conductive before the laminate isused as an ion-exchange membrane and, due to the coating, openings inthe weave are not needed to facilitate lamination of the fabric to film.Yarn counts of 1 to 50 yarns per centimeter, preferably from 25 to 40,can be used.

When knitted, any known method of knitting may be used. These includesingle, double, full fashion, tricot and raschel knitting. Gauge is notcritical for the same reasons as with woven fabric. Gauges up to 66,preferably 10 to 40, can be used.

While not preferred, it may be desirable to use soluble or degradablefibers, such as rayon or polyester, along with the functional fibers.They may be used because, after final hydrolysis and use of theassembled membrane when these fibers are solubilized or degraded, theresistance of the membrane will be decreased. Care should be taken,however, not to have the soluble or degradable fibers extend from onesurface to the other, or the non-porous membrane will become a porousdiaphragm and, in the case of a chloralkali cell, the caustic productwill contain too much salt.

To reduce its thickness, coated fabric may be heat set and/or calenderedbefore lamination with film to make membranes.

Membranes usually have an overall thickness of 50-250 micrometers,especially 125-200 micrometers.

The coated fabric can be melt-laminated with at least one film of atleast one melt-processible fluorinated cation exchange resin precursorin which the non-functional:functional group ratio is within threeunits, preferably one unit, of that of the sheath or coating resin tomake a membrane or multiple-membrane.

In a bimembrane, the fabric may be in the sulfonic or carboxylic layeror both, but preferably is in the sulfonic layer, which is usuallythicker. In place of fabric, non-woven fibrils can be used.

Membranes or bimembranes may be used flat in various known filter presscells, or may be shaped around an electrode. The latter is especiallyuseful when it is desired to convert an existing diaphragm cell to amembrane cell in order to make higher quality caustic.

Membranes can be swelled with polar solvents (such as lower alcohols oresters, tetrahydrofuran, or chloroform) and then dried, preferablybetween flat plates, to improve their electrolytic performance. Beforemounting in commercial cell support frames, which may be 1-3 meters on aside, the membrane can be swelled so that it will not wrinkle after itis clamped in the frame and exposed to electrolytic fluids. Among theswelling agents that can be used are water, brine, caustic, loweralcohols, glycols, and mixtures thereof.

The membranes described herein can also be modified on either surface orboth surfaces thereof so as to have enhanced gas release properties, forexample by providing optimum surface roughness or smoothness by hot rollembossing or by embossing with a porous paper. When embossing with aporous paper, a release paper can be applied to an outer surface of themembrane prior to passing through a laminator used, for example, toapply a reinforcement for the membrane. Such surface embossing isfurther described in U.S. Pat. No. 4,349,422. Preferably the resultingsurface roughness is about 2-5 micrometers as measured, for example, ona Bendix Model 1020 profilometer.

Cells can have two or three compartments, or even more. If three or morecompartments are used, the membrane is commonly used next to the cathodecompartment, and the other dividers may be porous diaphragms ormembranes based on polymers having pendant side-chains with terminal--CF₂ SO₃ -- ion-exchange groups only.

Bipolar or monopolar cells can be used. In ordinary use, the carboxylicside of the membrane will face the cathode. One can use (n) cells inseries, with anolyte flowing from first cell (1) to cell (n) andcatholyte flowing from cell (n) to cell (1). The cells may use identicalmembranes or different membranes may be used in different cells.Membranes using only polymers having pendant side chains with terminal--CF₂ SO₃ -- groups may be used in cell (n) and possibly others near it.Cell (n) may be two or more cells in parallel.

The membrane may be disposed horizontally or vertically in the cell, orat any angle from the vertical.

Any of the conventional electrodes or electrode configurations may beused. The anode for a chloralkali cell should be resistant to corrosionby brine and chlorine, resistant to erosion, and preferably shouldcontain an electrocatalyst to minimize chlorine overvoltage. Thewell-known dimensionally stable anode is among those that are suitable.A suitable base metal is titanium, and the electrocatalysts includereduced platinum group metal oxides (such as Ru, and the like) singly orin mixtures, optionally admixed with a reduced oxide of Ti, Ta, Nb, Zr,Hf, V, Pt, or Ir. The electrocatalysts may be heat treated forstability.

The anode may be a `zero-gap` anode, against which the membrane isurged, the anode being permeable to both liquids and gases.Alternatively, the anode may be kept a small distance from the membraneby the use of a spacer, against which the membrane is urged by a smallhydraulic head on the other side of the membrane. The spacer may be madeof a plastic which is resistant to the chemicals in the anolyte, such aspolytetrafluoroethylene, ethylene/tetrafluoroethylene copolymer, orpolychlorotrifluoroethylene. It is desirable that the spacer or theelectrode should have open vertical channels or grooves to facilitatethe escape of gas evolved at the anode.

Whether or not there is a spacer, it may be desirable to have the anodeopenings slanted so the gas is carried away from the membrane and sothat anolyte circulation past the membrane is maximized. This effect canbe augmented by using downcomers for anolyte which has been lifted bythe rising gas bubbles.

The anode may be a screen or a perforated plate or a powder, any ofwhich may be partially embedded in the anode surface layer of themembrane. When the anode is embedded, the current may be supplied to theanode by current distributors which contact the anode at numerousclosely-spaced points. The anode may be a porous catalytic anodeattached to or pressed against the membrane or attached to or pressedagainst a porous layer, which is in turn attached to or pressed againstthe membrane.

The cathode for a chloralkali cell should be resistant to corrosion bythe catholyte, resistant to erosion, and preferably contain anelectrocatalyst to minimize hydrogen overvoltage. The cathode may bemild steel, nickel, or stainless steel, for example, and theelectrocatalyst may be platinum black, palladium, gold, spinels,manganese, cobalt, nickel, Raney nickel, reduced platinum group metaloxides, alpha-iron and the like.

The cathode may be a `zero-gap` cathode, against which the membrane isurged, the cathode being permeable to both liquids and gases.Alternatively, the cathode may be kept a small distance from themembrane by the use of a spacer, against which the membrane is urged bya small hydraulic head on the other side of the membrane. In the case ofa three-compartment cell, both membranes may be urged against electrodesor spacers by a hydraulic head on the center compartment. The spacer maybe made of a plastic which is resistant to the chemicals in thecatholyte, such as polytetrafluoroethylene, ethylene/tetrafluoroethyleneresin, or polychlorotrifluoroethylene. It is desirable that the cathodespacer or electrode have open vertical channels or grooves to facilitatethe escape of gas evolved at the cathode.

Whether or not there is a spacer, it may be desirable to have thecathode openings slanted so the gas is carried away from the membraneand catholyte flow past the membrane is maximized. This effect may beaugmented by using downcomers for catholyte which has been lifted byrising gas bubbles. The cathode may be a porous cathode, pressed againstthe membrane or pressed against a porous layer, which is in turnattached to or pressed against the membrane.

An oxygen cathode can be used, in which oxygen is supplied to thecathode and substantially no hydrogen is evolved, with the result beinglower cell voltage. The oxygen may be supplied either by bubblingthrough the catholyte and against the cathode, or by feedingoxygen-containing gas through a porous inlet tube which also serves ascathode and is coated with electrocatalyst.

It has long been known that in the electrolysis of brine to makechlorine and caustic, it is desirable to use sodium chloride (NaCl)having low calcium (Ca) and magnesium (Mg) content (hardness). It isalso well known how to remove hardness from NaCl solutions to very lowlevels. Heavy metals (such as iron and mercury) and foreign anions (suchas iodide and sulfate) should also be substantially removed. Some of thecontaminants in make-up brine can be removed by passing the brinethrough a diaphragm cell before it is fed to the membrane cell system.Further hardness reductions can be achieved by passing the brine througha chelate ion exchanger, preferably one containing --NHCH₂ COOH groups,or a phosphate may be added to the brine to precipitate insoluble salts.

Brine fed to the cell is usually close to the saturation concentration,but lower brine concentration is acceptable. Brine leaving the anolytechamber may be as low as about 2 wt. % NaCl, but is more often 10-15 wt.% NaCl, or even higher.

Because a bimembrane or three-layer membrane has lower electricalresistance than an all-carboxylic membrane, it can be operated at lowervoltage or higher current density. Good results can be obtained at acurrent density of 1.0-7.0 kiloamperes per square meter (kA/m²),preferably 3.0-5.0 kA/m².

It is desirable to acidify the anolyte to minimize the formation ofoxygen and chlorate at the anode.

Anolyte acidity is normally adjusted to a value in the range of pH 1-5by addition of hydrochloric acid or hydrogen chloride to the recyclebrine. Recycle brine may be concentrated by addition of solid saltand/or by evaporating or distilling water from the stream.

While membrane cells are frequently operated at approximatelyatmospheric pressure, there can be advantages to operating them atelevated pressure. While direct current is ordinarily used in membranecells, one can also use pulsed direct current or half-wave AC orrectified AC or DC with a square wave.

Chloralkali synthesis is normally carried out at about 70°-100° C. Thecatholyte can be kept 5°-20° cooler than the anolyte temperature.

In any of the above arrangements, either or both of the electrodes canhave a catalytically active surface layer of the type known in the artfor lowering the overvoltage of an electrode. Such electrocatalyst canbe of a type known in the art, such as those described in U.S. Pat. Nos.4,224,121 and 3,134,697, and UK No. 2,009,788A. Preferred cathodicelectrocatalysts include platinum black, Raney nickel and rutheniumblack. Preferred anodic electrocatalysts include platinum black andmixed ruthenium and titanium oxides.

There are several methods by which the sheath/core yarn of the presentinvention may be made.

Single or multiple coating steps with a solution or dispersion or otherliquid composition of the copolymer in a form other than the alkalimetal salt may be used. Among the known liquid compositions are those ofthe sulfonic acid form (UK No. 1,286,859 or U.S. Pat. No. 4,433,082),the --SO₂ F form (U.S. Pat. No. 4,348,310 and U.S. Pat. No. 4,650,551),and the --COOCH₃ form (U.S. Pat. No. 4,348,310 and Japanese Laid-openApplication No. J55/149336) and the --COOH form (U.S. Pat. No.4,385,150). Temperature, immersion time, and solution concentration arenot critical, though multiple immersions may be needed if the liquidcomposition has a low polymer concentration or viscosity. The thicknessof the sheath made by this process may be 2.5-13 micrometers, preferably5-8 micrometers.

In the case where the yarn has --COOalkali metal or --SO₃ alkali metalend groups, the outer surface can be converted to the free acid form bycontacting the yarn with 5-10 wt. % aqueous mineral acid, such as HCl orHNO₃, for a time and temperature sufficient to convert the outer surfaceof the yarn to more melt-processible form, that is to --COOH or --SO₃ Hgroups, respectively. The time can be short, such as 15 minutes atelevated temperatures as in a steam bath, or longer at lowertemperatures. The time, temperature, and acid concentration can bevaried to control penetration into the yarn so that the yarn will bemelt-processible on the surface and hydrolyzed within. The conditionswill vary with yarn composition, denier and whether it is a monofilamentor a multifilament. The degree of conversion can be determined bycutting a cross-section of a sample of the yarn and staining with acationic dye.

While the --COOH and SO₃ H groups are more melt-processible than theiralkali metal counterparts, they are so with difficulty.

In the case of the --COOH groups, it is preferable to make the outersurface more melt-processible (lower viscosity) and more compatible withthe --COOR films used in the lamination step by esterification into the--COOR form, R being a lower alkyl (C₁₋₅). Esterification conditions areknown to those skilled in the art, specifically being taught in U.S.Pat. No. 4,415,679 which is incorporated by reference.

The --SO₃ H form may be converted to a more melt-processible form, suchas the --SO₂ Cl form, using reaction conditions taught in Example 1 ofU.S. Pat. No. 4,151,053 which is incorporated by reference; or it may beconverted to a melt-processible salt by treatment with a tertiary amineor its salt, or a quaternary ammonium base or its salt using reactionconditions taught in U.S. Pat. No. 3,884,885 which is incorporated byreference. In a variation on this process, the alkali metal salt of thesulfonyl yarn may be converted on the surface to a melt-processible saltby treatment with a tertiary amine or its salt or a quaternary ammoniumbase or its salt.

Preferably, after conversion of the surface to a more melt-processibleform, the core will still be sufficiently oriented and hydrolyzed tohave a tenacity of at least 0.5 grams per denier and a tensile modulusof at least 2 grams per denier.

The coated cation exchange fabric may be made from the sheath/core yarn,preferably by weaving or knitting. The weave used in the fabric is notcritical, and the maximum yarn count (tightness of weave) is notcritical as discussed above.

Alternatively, it may first be made from oriented and hydrolyzed cationexchange resin in yarn or other form followed by a modification orcoating of one of both of its surfaces employing the same processes asdescribed above for yarn.

The fabric may be calendered or heat set. An advantage of calenderingwith heat and light pressure or in any way applying heat and lightpressure to the fabric while it is on the loom or immediately after itleaves the loom is that the yarns fuse lightly to one another,stabilizing the fabric against shifting of yarns during handling priorto lamination. The temperature of this step should be above roomtemperature but below the melting point of the melt-processible coatingresin; an effective temperature and pressure can be determined routinelyfor the process chosen. The pressure should be 1-100 kPa, depending onthe process and temperature used. Pressure and heat can be applied usingheated calender rolls, with or without release paper or film between theheated roll and the fabric. In a batch process, heated platens may bepressed lightly against the fabric on the loom or immediately afterremoval from the loom, optionally using a release paper or film betweenthe platen and the fabric.

The method of laminating the cation exchange precursor film or films tothe cation exchange fabric is not critical. Several methods have beendisclosed in the art, including that used in the Examples.

Preferably, the gas release properties of the membranes are enhanced byproviding thereon a gas- and liquid-permeable porous non-electrodelayer. Such non-electrode layer can be in the form of a thin hydrophiliccoating or spacer and is ordinarily of an inert electroinactive ornon-electrocatalytic substance. Such non-electrode layer should have aporosity of 10 to 99%, preferably 30 to 70%, and an average porediameter of 0.01 to 2000 micrometers, preferably 0.1 to 1000micrometers, and a thickness generally in the range of 0.1 to 500micrometers, preferably 1 to 300 micrometers. A non-electrode layerordinarily comprises an inorganic component and a binder; the inorganiccomponent can be an inorganic compound which is chemically stable in hotconcentrated caustic and chlorine, and can be of a type as set forth inUK No. 2,064,586, preferably tin oxide, titanium oxide, zirconium oxide,or an iron oxide such as Fe₂ O₃ or Fe₃ O₄. Other information regardingnon-electrode layers on ion-exchange membranes is found in publishedEuropean Patent Application No. 31660, and in Japanese Laid-open PatentApplications Nos. 56-108888 and 56-112487. The particle size of theinorganic material can be about 1-100 micrometers, and preferably 1-10micrometers.

The binder component in a non-electrode layer can be, for example,polytetrafluoroethylene, a fluorocarbon polymer at least the surface ofwhich is hydrophilic by virtue of treatment with ionizing radiation inair or a modifying agent to introduce functional groups such as --COOHor --SO₃ H (as described in published UK Patent Application GB No.2,060,703A) or treatment with an agent such as sodium in liquid ammonia,a functionally substituted fluorocarbon polymer or copolymer which hascarboxylic or sulfonic functional groups, or polytetrafluoroethyleneparticles modified on their surfaces with fluorinated copolymer havingacid type functional groups (GB No. 2,064,586). Such binder can be usedin an amount of about from 10 to 50 wt. % of the non-electrode layer orof the electrocatalyst composition layer. In addition to the inorganiccomponent and the binder, the dispersion used to apply the inorganiccomponent can include a thickener such as methyl cellulose or polyvinylalcohol and a small amount of nonionic surfactant.

Composite structures having non-electrode layers thereon can be made byvarious techniques known in the art, which include preparation of adecal which is then pressed onto the membrane surface, spray applicationof a slurry or a liquid composition (for example, dispersion orsolution) of the binder followed by drying, screen or gravure printingof compositions in paste form, hot pressing of powders distributed onthe membrane surface, and other methods as set forth in British PatentNo. 2,064,586 or Japanese Laid-open Patent Application No. J57/89490.Such structures can be made by applying the indicated layers ontomembranes in melt-fabricable form, and by some of the methods ontomembranes in ion-exchange form; the polymeric component of the resultingstructures when in melt-fabricable form can be hydrolyzed in knownmanner to the ion-exchange form.

Membranes which carry thereon one or more non-electrode layers can beemployed in an electrochemical cell regardless of the distances betweenthe anode, the membrane and the cathode. That is, they are useful in socalled finite-gap, narrow-gap and zero-gap configurations.

EXAMPLES Example 1

A copolymer of TFE and CF₂ ═CFOCF₂ CF(CF₃)OCF₂ CF₂ SO₂ F with a ratio ofthe two monomers of 6.6:1 was melt spun and melt drawn downward at atemperature of 300° C. through a 6-hole spinneret with a takeoff speedof 75 meters per minute (m/min). The yarn was drawn at a rate of 175m/min. at 300° C., resulting in a 233% elongation. The tensileproperties of both drawn and undrawn ion-exchange yarns were determined:

    ______________________________________                                                          Drawn Undrawn                                               ______________________________________                                        Tenacity (g/denier) 0.70    0.28                                              Modulus (g/denier)  1.1     0.5                                               Orientation angle (degrees)                                                                       15.1    18.3                                              Apparent crystal size (nm)                                                                        5.3     6.2                                               Density (g/cm.sup.3)                                                                              2.004   2.005                                             ______________________________________                                    

Samples of both yarns were wound on an inert porous support andhydrolyzed overnight in a solution of 10% KOH, 30% dimethylsulfoxide,and 60% water. The yarns were then rinsed in water, dried in air, andtested for tensile properties:

    ______________________________________                                                          Drawn Undrawn                                               ______________________________________                                        Tenacity (g/denier) 0.71    0.38                                              Modulus (g/denier)  5.0     3.2                                               Orientation Angle (degrees)                                                                       26.5    33.3                                              Apparent Crystal Size (nm)                                                                        3.1     3.0                                               Density (g/cm.sup.3)                                                                              1.830   1.862                                             ______________________________________                                    

These experiments show that tenacity is about doubled by melt drawing1×, and modulus is increased 5-6 fold by hydrolysis to the potassiumsalt form.

Example 2

The drawn, hydrolyzed yarn made in Example 1 was coated with a 10%solution of a similar copolymer with TFE:comonomer ratio of 3.36:1 inCF₂ ClCFCl₂, using 5 passes and obtaining partial coverage. This samplewas labeled "Example 2". After hydrolysis was repeated, the fullyhydrolyzed yarn was labelled "Fully Hydrolyzed". The tensile propertieswere determined:

    ______________________________________                                                     Example 2                                                                             Fully Hydrolyzed                                         ______________________________________                                        Tenacity (g/denier)                                                                          0.82      0.71                                                 Modulus (g/denier)                                                                           6.1       5.4                                                  Density (g/cm.sup.3)                                                                         1.862     1.862                                                ______________________________________                                    

Also, using thermal mechanical analysis, it was found that thesheath/core yarn contracts much less than all-melt-processable yarn atthe temperature of lamination.

Example 3

The coated yarn of Example 2 was woven into a fabric with an averageyarn density of 1.2 yarns/cm in the warp direction and 1.2 yarns/cm inthe weft direction. The fabric was laminated into a membrane by placingthe fabric on a 25 micrometer film of the same polymer with aTFE:functional monomer mole ratio of 6.58:1; placing on the fabric amelt-coextruded film containing a 100 micrometer layer of the samesulfonyl polymer and a 38 micrometer layer of a 6.4:1 TFE:CF₂ ═CFOCF₂CF(CF₃)OCF₂ CF₂ COOCH₃ copolymer; placing the sandwich, ester side up,on a sheet of porous release paper; and applying a vacuum of 71 kPa for15 seconds while the upper surface is heated radiantly at 225° C.

A control was made which was substantially the same except the fabricwas made of PTFE yarn of 200 denier. Both membranes were hydrolyzed inKOH/DMSO/water at 90° C., washed in water, preswelled in 2 wt. % NaOH,and installed in identical laboratory cells with an effective membranediameter of about 7.5 cm. The cathode was a mild steel, the anode wasactivated with a coating containing ruthenium oxide, and the membranewas urged against the anode by catholyte head. Purified brine was used.Electrolysis was carried out at 90° C. and 3.1 kA/m² current density tomake 32 wt. % NaOH. The sample had lower voltage and the control hadhigher caustic current efficiency. The results in the following table(power consumption in kilowatt hours/metric ton of sodium hydroxide)show a desirable lower power consumption was achieved versus thecontrol.

    ______________________________________                                                          Sample Control                                              ______________________________________                                        Days on line        14       21                                               Final power consumption                                                                           2410     2445                                             (kWH/MT)                                                                      Average of daily readings                                                                         2407     2441                                             after level operation,                                                        power consumption                                                             Final current efficiency (%)                                                                      95.7     97.0                                             Range of daily readings,                                                                          94.6-97.0                                                                              91.1-98.5                                        current efficiency (%)                                                        Final cell voltage  3.44     3.54                                             Range of cell voltage                                                                             3.42-3.45                                                                              3.53-3.60                                        ______________________________________                                    

Example 4

Example 2 is repeated, except the sulfonyl polymer used in the yarn andthe sulfonyl films is made from CF₂ ═CFOCF₂ CF₂ SO₂ F (see a paper byEzzell et al. presented at the AIChE meeting in Houston on 3/27/85) andhas a mole ratio of 11.8:1. The yarn is laminated with a 75 micrometerfilm of the same copolymer, total thickness of this copolymer in thelaminated membrane is only 75 micrometers. The hydrolyzed yarn is dipcoated with a 1,2-dibromotetrafluoroethane solution of a polymer madefrom the same monomers, as described in Example 1 of U.S. Pat. No.4,650,551. After drying, the coated yarn is woven into a fabric as inExample 3 except the yarn count is 3 yarns/cm in both warp and weftdirections. The fabric is stabilized by placing it between sheets ofrelease paper and pressing at 10 kPa at 150° C. in a press. This makesthe fabric stable, even during manual handling. The stabilized fabric islaminated and used in electrolysis as in Example 3. The voltage is lowerthan that of a control with PTFE reinforcement of 200 denier.

Example 5

Example 4 is repeated, except the sulfonyl polymer used in the yarn andthe sulfonyl films is made from TFE:CF₂ ═CFO[CF₂ CF(CF₃)O]₂ OCF₂ CF₂ SO₂F and has a mole ratio of 2.8:1, and the hydrolyzed, oriented core yarnis coated with a liquid composition according to Example 2 of U. S. Pat.No. 4,433,082. The current efficiency and voltage are about the same asthose in Example 2.

Example 6

Example 3 was repeated except the film on the cathode side was 50micrometers thick and had a TFE:CF₂ ═CFOCF₂ CF(CF₃)OCF₂ CF₂ SO₂ F ratioof 9.1:1 and that on the anode side was 125 micrometers thick and had aTFE:CF₂ ═CFOCF₂ CF(CF₃)OCF₂ CF₂ SO₂ F ratio of 6.6:1. The control wasidentical to the sample, except the reinforcement was made of PTFE yarnof 200 denier. The electrolysis test was the same except the NaOHconcentration was 20 wt. %, still a little too high for optimumperformance of this membrane. The results were:

    ______________________________________                                                        Sample Control                                                ______________________________________                                        Days on line      11       11                                                 Final caustic current                                                                           72.2     71.6                                               efficiency (%)                                                                Range of daily readings,                                                                        70.4-72.6                                                                              63.8-71.9                                          current efficiency (%)                                                        Final cell voltage                                                                              3.49     3.59                                               Range of daily cell                                                                             3.37-3.49                                                                              3.39-3.59                                          voltages                                                                      ______________________________________                                         This example demonstrates the invention in an allsulfonic membrane.      

Example 7

Example 3 is repeated except the yarn density is 30 yarns/cm in each ofthe warp and weft directions. The cell voltage is substantially the sameas in Example 3.

We claim:
 1. A fluorinated cation exchange membrane comprising alamination of:(A) a fabric comprising interengaged oriented, hydrolzedfluorinated cation exchange yarns, the fabric base being a copolymerhaving a mole ratio of non-functional:functional monomer of 2.8-11.8 to1, the fabric base being coated on at least one surface or theindividual yarns of the fabric being coated or the individual yarns ofthe fabric having been superficially converted to melt-processible formprior to fabric formation therefrom, the coating or superficialconversion comprising a melt-processible form of a fIuorinated cationexchange copolymer having a mole ratio of non-functional:functionalmonomer differing from that of the fabric base copolymer by no more than3 units; and (B) at least one film of at least one hydrolyzableprecursor to a melt-processible fluorinated cation exchange copolymer inwhich the mole ratio of non-functional:functional monomer is 2.8-11.8 to1, said mole ratio in the film adjacent to the fabric being differentthan that of the fabric surface copolymer by no more than 3 units. 2.The membrane of claim 1 wherein the fabric base is composed of knittedor woven yarn comprising a fluorinated cation exchange copolymer havinga mole ratio of non-functional:functional monomer of about 2.8-11.8:1,the yarn being oriented, hydrolyzed and having a tenacity of at least0.5 grams per denier, a tensile modulus of at least 2 grams per denierand a denier of between 50 and
 400. 3. The membrane of claim 1 whereinthe fabric base is composed of knitted or woven yarn comprising afluorinated cation exchange copolymer having a mole ratio ofnon-functional:functional monomer of about 2.8-11.8:1, the yarn beingoriented, hydrolyzed and having a tenacity of at least 0.5 grams perdenier, a tensile modulus of at least 2 grams per denier, a denier ofbetween 50 and 400; the coating having been applied to the yarn beforeits having been woven or knitted, the coating comprising amelt-processible form of a fluorinated cation exchange copolymer havinga mole ratio of non-functional:functional monomer differing from that ofthe more than 3 units.
 4. The cation exchange membrane of claim 1wherein the mole ratio of non-functional:functional monomer in thefabric base copolymer and in the coating copolymer differ by no morethan 1 unit.
 5. The cation exchange membrane of claim 4 wherein thefabric is calendered.
 6. The cation exchange membrane of claim 2 whereinthe fabric has a yarn count of 1 to 50 yarns per centimeter if woven ora gauge of up to 66 if knitted.
 7. The cation exchange membrane of claim6 wherein the yarn count is 25 to 40 yarns per centimeter or the gaugeis 10 to
 40. 8. The membrane of claim 1 wherein the mole ratio ofnon-functional:functional monomer in the copolymer of (A) is 5.3-7.5 to1 and the difference between that of the copolymer of the film adjacentto the fabric that of the fabric surface copolymer is no more than 1unit.
 9. The membrane of claim 2 wherein the mole ratio ofnonfunctional:functional monomer in the copolymer of (A) is 5.3-7.5 to 1and the difference between that of the copolymer of the film adjacent tothe fabric that of the fabric surface copolymer is no more than 1 unit.10. The membrane of claim 3 wherein the mole ratio ofnon-functional:functional monomer in the copolymer of (A) is 5.3-7.5 to1 and the difference between that of the copolymer of the film adjacentto the fabric that of the fabric surface copolymer is no more than 1unit.
 11. The membrane of claim 1 wherein the functional monomer in thecopolymer of (B) is a fulfonyl monomer, a carboxyl monomer or acombination of sulfonyl and carboxyl monomers and the non-functionalmonomer copolymer of (B) is tetrafluoroethylene.
 12. The membrane ofclaim 11 in which the fabric contains only sulfonic ion-exchange groupsfor both the base and the coating structures, the film adjacent to thefabric containing substantially sulfonic ion-exchange groups in whichTFE:functional group ratio is within one unit off that of the coatingcopolymer and the catholyte-facing film surface containing onlycarboxylic ion-exchange groups.
 13. The membrane of claim 11 wherein themembrane is a carboxyl/sulfonyl bimembrane having a carboxyl layer and asulfonyl layer, the coated fabric having sulfonyl monomer as thefunctional monomer adhered in the sulfonyl layer.
 14. The membrane ofclaim 11 wherein the fabric is made from coated yarn made of ahydrolyzabole precursor of the fluorinated cation exchange resin havingcarboxyl monomer as the functional monomer, and the film is made of ahydrolyzable precursor of the fluorinated cation exchange copolymerhaving carboxyl monomer as the functional monomer.
 15. An improvedprocess for the electrolysis of an alkali metal halide to make a halogenand an alkali metal hydroxide, the improvement comprising use of themembrane of claim
 1. 16. An improved process for the electrolysis of analkali metal halide to make a halogen and an alkali metal hydroxide, theimprovement comprising use of the membrane of claim
 2. 17. An improvedprocess for the electrolysis of an akali metal halide to make a halogenand an alkali metal hydroxide, the improvement comprising use of themembrane of claim
 3. 18. An improved process for the electrolysis of analkali metal halide to make a halogen and an alkali metal hydroxide, theimprovement comprising use of the membrane of claim
 6. 19. An improvedprocess for the electrolysis of an alkali metal halide to make a halogenand an alkali metal hydroxide, the improvement comprising use of themembrane of claim
 7. 20. An improved process for the electrolysis of analkali metal halide to make a halogen and an alkali metal hydroxide, theimprovement comprising use of the membrane of claim 13.